force layout with d3.timer instead of tick loop

force.reqanimframe.js

var force = d3.layout.force()
  .charge(-150)
  .linkDistance(30)
  .size([width, height]);

d3.json("assets/500nodes.json", function(error, graph) {
  if (error) throw error;

  // Task 2:
  // Connect the force layout to the nodes and links in our dataset
  // and start the simulation
  force
    .nodes(graph.nodes)
    .links(graph.edges)
    .start();

  // Task 3:
  // Render the links connecting the nodes. They should be 'line' elements
  // that have a stroke-width that is the square root of the link 'value'
  // attribute, representing the strength of the relationship. They should bind
  // to the graph edges array.
  var link = svg.selectAll(".link")
    .data(graph.edges)
    .enter()
      .append("line")
      .attr("class", "link")
      .style("stroke-width", function(d) {
        return Math.sqrt(d.value);
      });
  
  // Create node elements for each node. They are circle elements with a fixed
  // radius of 5. They are colored by their group association and have
  // drag enabled by default.
  var node = svg.selectAll(".node")
      .data(graph.nodes)
    .enter().append("circle")
      .attr("class", "node")
      .attr("r", 5)
      .style("fill", function(d) { return color(d.group); })
      .call(force.drag);

  // with reqAnimframe:
  function animate() {
    link
      .attr("x1", function(d) { return d.source.x; })
      .attr("y1", function(d) { return d.source.y; })
      .attr("x2", function(d) { return d.target.x; })
      .attr("y2", function(d) { return d.target.y; });

    node
      .attr("cx", function(d) { return d.x; })
      .attr("cy", function(d) { return d.y; });
  }

  d3.timer(animate);